KR102332304B1 - Improvements relating to propylene oxide purification - Google Patents

Abstract

The present invention relates to a process for separating impurities from impure PO, comprising distilling the impure PO in a distillation zone to provide a distillate PO of improved purity, wherein heat is provided by bottom reboiler and intermediate reboiler, , intermediate reboilers provide heat at a lower temperature than bottom reboilers. Suitable distillation systems are also disclosed.

Description

IMPROVEMENTS RELATING TO PROPYLENE OXIDE PURIFICATION

The present invention relates to the purification of propylene oxide (PO). In particular, but not exclusively, the present invention relates to an energy efficient method and system for such purification.

PO is an essential building block for a variety of chemicals and products. Worldwide production of PO exceeds 7 million tonnes per year.

Direct oxidation of propylene using air or oxygen to form PO tends to give low yields. Thus, PO is most commonly prepared with the aid of chemical mediators.

One known process comprises contacting an organic hydroperoxide and propylene with a heterogeneous epoxidation catalyst and discharging a product stream comprising PO and alcohol. A specific organic hydroperoxide that can be used in this epoxidation process is ethylbenzene hydroperoxide (EBHP), in which case the alcohol obtained is 1-phenylethanol. 1-Phenylethanol can be converted to styrene by dehydration. EBHP can be prepared by reaction of ethylbenzene with oxygen.

Another known method for the production of PO is the co-production of PO and methyl tert-butyl ether (MTBE). This process includes reaction steps similar to those described above for preparing styrene/PO. In the epoxidation step, tert-butyl hydroperoxide is reacted with propylene to form PO and tert-butanol. The tert-butanol is then etherified with MTBE.

Another known method involves preparing PO with the aid of cumene. In this process, cumene is reacted with oxygen or air to form cumene hydroperoxide. The cumene hydroperoxide thus obtained is reacted with propylene in the presence of an epoxidation catalyst to produce PO and cumyl alcohol. The latter can be converted to cumene with the aid of a heterogeneous catalyst and hydrogen.

According to recent developments, it is also known to prepare PO from propylene with the aid of hydrogen peroxide as a vehicle.

Regardless of the particular epoxidation method used, the PO product generally requires purification to remove by-products and impurities. In fact, in most applications, it is important to reduce impurities in PO to very low levels.

Some by-products of the epoxidation process can be easily isolated by distillation. However, epoxidation processes also tend to form by-products and impurities that are more difficult to separate. When, for example, the epoxidation is carried out with an organic hydroperoxide, the organic hydroperoxide is mostly reduced to the corresponding alcohol, which tends to be easy to separate. However, small amounts of other oxygen-containing compounds such as methanol, acetone, acetaldehyde, propionaldehyde, etc., as well as hydrocarbons, are also produced, which are difficult to separate and often remain as impurities in the PO product even in subsequent conventional distillations. Hydrocarbon impurities associated with PO are believed to be propylene derivatives having 4 to 7 carbon atoms per molecule, in particular derivatives having 6 carbon atoms per molecule. C ₆ compounds mainly include methyl pentene and methyl pentane. Other epoxidation methods also form impurities of the same or similar structure that are similarly difficult to separate.

The separation of typical impurities in PO tends to require multiple distillation steps. Additionally, final purification (or finishing) to high purity by distillation typically requires very large sized columns, especially when the relative volatilities of impurities are low compared to PO.

It is known to use extractive distillation techniques to help separate impurities with low relative volatilities. For example, US-A-3909366 describes the purification of propylene oxide by extractive distillation in the presence of aromatic hydrocarbons having 6 to 12 carbon atoms, such as ethyl benzene. For example, cyclic paraffin (see US-A-3464897), lower glycols (see US-A-3578568), water (see US-A-4140588), t-butyl alcohol (see US-A-5006206) and heptane Various other extractive distillation solvents have also been proposed, including However, such methods still typically require large sized columns.

US-A-5772854 relates to the use of so-called "paired" reboilers, ie serially connected reboilers, in the purification of propylene oxide. Specifically, US-A-5772854 discloses that propylene contaminated with water, methanol and acetone in an extractive distillation column in the presence of an oxyalkylene glycol extractive distillate, under distillation conditions selected to promote the formation and maintenance of an acetone buffer in the distillation column. A method of purifying an oxide feedstock is provided, wherein a higher boiling (heavier) distillation fraction comprising substantially all of an oxyalkylene glycol, water and acetone is continuously withdrawn from the distillation column, the higher boiling point The (heavier) distillation fraction of is partially evaporated in the first reboiler; The remaining liquid is partially evaporated in the second reboiler and the vapor is recycled to the extractive distillation column.

GB-A-1549743 relates to a method of controlling the heat input to the reboiler section of a distillation column to improve the separation efficiency to achieve the desired end product.

In GB-A-1549743, "reboiler section" is defined on page 1, lines 62 to 64 as being the portion of the column below the lowest stage. Thus, the so-called "reboiler section" as described in GB-A-1549743 is the bottom section in the distillation column.

The method of GB-A-1549743 includes discharging a liquid bottoms stream from a partially split reboiler section of a distillation column, introducing a first portion of liquid bottoms stream material into a first reboiler, said first reboiler introducing the mixed-phase bottoms stream material produced in the substantially liquid-free region of the reboiler section, introducing a second portion of the liquid bottoms stream material into a second reboiler, and in the second reboiler introducing the resulting mixed-phase bottoms stream material into a substantially liquid-free region of the same reboiler section as the mixed-phase bottoms stream material produced in the first reboiler.

Thus, in the process of GB-A-1549743, the liquid bottoms stream material is withdrawn from the section below the lowest stage in the distillation column, and the mixed bed bottoms streams from the first reboiler and the second reboiler are also at the same location in the distillation column. , ie it will be returned to the bottom of the lowest stage in the distillation column.

This is clearly shown in the drawing of GB-A-1549743, where the liquid bottoms stream material exits the so-called reboiler section via an outlet port 25 and the mixed phase bottoms stream from the first reboiler and the second reboiler. is also returned through inlet ports 30 and 34 in the reboiler section. The lowest level in the figure is (11).

Purification of PO by distillation, and production of PO as a whole, is very energy intensive, especially given the large size of the required column. It is an object of the present invention to provide a method and system for the separation of impurities from PO, wherein energy saving is also allowed.

It has now been found that the vapor pressure of PO can be used to enable energy savings during PO production and purification.

From a first aspect, the present invention relates to a method for separating impurities from impure PO, comprising distilling the impure PO in a distillation zone to provide distillate PO of improved purity, wherein the heat is transferred to a bottom reboiler and an intermediate reboiler. Provided to the distillation zone by reboiler, the intermediate reboiler provides heat at a lower temperature than the bottom reboiler.

Because the intermediate reboiler provides heat at a lower temperature, a wider range of heat sources can be used in the intermediate reboiler. In particular, lower temperature requirements allow the use of low grade heat sources that are often available in the context of PO production and purification and would otherwise be discarded.

In an embodiment, the heat provided by the intermediate reboiler is derived from a heat source, for example low pressure (LP) steam having a temperature in the range from 80 to 180°C, in particular from 120 to 140°C. The LP steam may for example have a pressure in the range of 1.0 bar abs to 10 bar abs, such as 1.2 bar abs to 3 bar abs.

In an embodiment, the heat provided by the intermediate reboiler is obtained directly or indirectly from a process stream that is cooled as part of a process for making PO. For example, the LP steam may be flash steam emitted from hot condensate. The hot condensate may be, for example, a condensate formed during an epoxidation process to produce PO, or during PO distillation. Thus, the heat provided by the intermediate reboiler can be derived from flash steam produced by cooling the process stream as part of the PO production process. An example of indirect heat from a process stream is heat from a heat pump system.

Advantageously, the LP steam can be freed by one or more energy saving measures during epoxidation or distillation. It is generally easier to save LP steam than higher grades of steam, such as medium pressure (MP) steam and high pressure (HP) steam. The MP steam may have a pressure in the range of, for example, 10 bar abs to 20 bar abs. The HP steam can have a pressure in the range of, for example, 20 bar abs to 100 bar abs. The method of the present invention advantageously enables energy saving measures by providing effective use of such liberated LP steam.

The distillation zone may be configured in any manner suitable for separating impurities from impure PO, ie purifying PO. To prevent accumulation of impurities in the distillation zone, one or more streams rich in impurities are typically removed from the distillation zone. The removed impurity stream(s) may include one or more of a bottoms impurity stream, a side-draw impurity stream, and an overhead impurity stream.

In an embodiment, the method comprises refluxing at least a portion of the overhead vapor present in the distillation zone. Depending on the design of the distillation zone and the level of impurities to be removed, the reflux ratio may range, for example, from 1 to 5. As is known in the art, reflux can help improve the purity of the distillation product. However, for energy efficiency, it is desirable to minimize the amount of reflux.

It has been recognized that in the distillation of PO, in particular the temperature and pressure of the distillation zone can be consistently balanced with both preferably high purification levels and preferably low reflux, while enabling energy savings with the aid of an intermediate reboiler.

In an embodiment, in order to provide a balance between low distillation zone temperatures and other process considerations, the pressure in the distillation zone is in the range of 1 bar abs to 10 bar abs, preferably in the range 1.5 bar abs to 5 bar abs, more preferably It can be maintained in the range of 2 bar abs to 3 bar abs.

The temperature of the distillation zone is influenced by the operation and configuration of the bottom reboiler and intermediate reboiler. In an embodiment, the bottom reboiler inputs heat to the bottom region of the distillation zone and the intermediate reboiler inputs heat to the top region of the distillation zone. The upper zone may be understood as any part of the distillation zone above the level at which heat is introduced by means of a bottom reboiler. Correspondingly, the bottom zone can be understood as the rest of the distillation zone, ie the level at which the bottom reboiler puts heat and the zone below it.

Advantageously, the temperature profile of the distillation zone may enable energy savings. (i) the upper region comprises a plurality of stages conforming to a relatively flat temperature profile; (ii) when a distillation zone is configured such that there is a sharp temperature difference between said plurality of stages in the bottom region and said plurality of stages in the top region, the energy saving opportunities resulting from the supply of heat by both bottom reboilers and intermediate reboilers are generally is improved

An upper region having a plurality of stages conforming to a relatively flat temperature profile has the opportunity to provide a lot of heat at a relatively constant temperature. Due to the vapor pressure and purity of the PO in this region of the zone, there is a relatively constant temperature, which may allow the use of a lower grade heat source. Thus, temperature and pressure can be controlled to provide a good balance between product purity and energy saving opportunities.

In an embodiment, the upper region of the distillation zone comprises a plurality of successive stages having different stage temperatures at most 40°C, preferably at most 30°C, more preferably at most 20°C. Suitably, said plurality of stages may comprise at least 10 theoretical plates, preferably at least 20 theoretical plates, more preferably at least 30 theoretical plates. In an embodiment, the temperature of said plurality of stages is in the range of 50 to 120 °C, preferably in the range of 55 to 100 °C, more preferably in the range of 60 to 90 °C. Advantageously, said plurality of stages may be positioned above the heat input of the intermediate reboiler.

The abrupt temperature difference between the stage and the bottom region conforming to a flat temperature profile in the upper region minimizes the inherent efficiency trade-off in providing heat from both intermediate and bottom reboilers. can help In particular, the sharp temperature difference allows the intermediate reboiler to be placed closer to the bottom reboiler, minimizing the distillation zone between the bottom reboiler and the intermediate reboiler, where the bottom reboiler duty is A reduced share of has an effect.

In an embodiment, at least one at least reference stage of the top region has a temperature at least 30°C, preferably at least 50°C, more preferably at least 70°C lower than the maximum stage temperature of the bottom region. Preferably, the reference stage is the lowest single of a plurality of successive stages of the upper region, for example as defined above, having a stage temperature that differs at most 40°C, preferably at most 30°C, more preferably at most 20°C. can Preferably, there may be a maximum of 30 theoretical plates, more preferably a maximum of 15 theoretical plates or a maximum of 10 theoretical plates between the reference stage and the bottom region. In an embodiment, the number of theoretical plates between the reference stage and the bottom region is at most 40%, preferably at most 20% or even at most 10% of the total number of theoretical plates in the distillation zone.

The intermediate reboiler may provide heat to any portion of the upper region of the distillation zone. A lower position within the upper region and/or a lower position of a plurality of stages with a relatively flat temperature profile is preferred. Advantageously, the intermediate reboiler can provide heat below the impure PO inlet of the distillation zone.

In an embodiment, the process comprises flowing a distillation mixture from an upper region of a distillation zone to an intermediate reboiler, heating the mixture by heat exchange with a heat source or medium, and heating the mixture, typically steam or vapor and liquid. and returning to the distillation zone as a combination. The particular location of the intermediate reboiler, ie its outlet (or outlet) and return, can be selected, for example, on the basis of the available heat source and the particular temperature profile desired in the distillation zone.

Advantageously, the intermediate reboiler comprises at least one stage of the upper region having a stage temperature at least 30° C., preferably at least 50° C., more preferably at least 70° C. lower than the maximum temperature of the bottom region, i.e. for example said Heat may be input to the distillation zone in a reference stage as defined in In an embodiment, the intermediate reboiler is advantageously a plurality of stages in the upper region having different stage temperatures, for example as defined above, at most 40°C, preferably at most 30°C, more preferably at most 20°C. Heat is applied at or below the lowest stage.

In an embodiment, the process comprises the steps of exiting the distillation mixture from the stage of the distillation zone having a temperature in the range of 50 to 120 °C, preferably in the range of 55 to 100 °C, more preferably in the range of 60 to 90 °C into an intermediate reboiler, heating the mixture and returning the mixture to the distillation zone.

In an embodiment, the process comprises the steps of exiting the distillation mixture from the distillation zone to an intermediate reboiler, heating the mixture, and subjecting the heated mixture to in the range from 50 to 120°C, preferably in the range from 55 to 100°C, more preferably in the range of returning to the stage of the distillation zone having a temperature in the range of 60 to 90°C.

In an embodiment, the distillation mixture exits from the top of the same stage of the distillation zone and is returned thereto.

The intermediate reboiler may take up any share of the total reboiler load of the distillation zone, thus reducing the load of the bottom reboiler. In an embodiment, heat is provided to the distillation zone by a plurality of intermediate reboilers, for example at least two, or at least three intermediate reboilers. In an embodiment, the intermediate reboiler(s) make up at least 10%, preferably at least 25%, more preferably at least 50% of the total reboiler load. In an embodiment, the intermediate reboiler(s) make up at most 80%, preferably at most 70% of the reboiler load.

In an embodiment, the intermediate reboilers or each of them have a load of at least 2 megawatts (MW), preferably at least 4 MW, more preferably at least 7 MW. In an embodiment, or each of the intermediate reboilers has a load of at most 12 MW, such as at most 10 MW.

The bottom reboiler holds the remaining share of the total reboiler load. In an embodiment, heat is provided to the distillation zone by a plurality of bottoms reboilers, for example at least two, or at least three bottoms reboilers. In an embodiment, or each of the bottom reboilers has a load in the range of 4 MW to 15 MW, such as in the range of 5 to 10 MW.

In an embodiment, the PO is distilled by extractive distillation. Accordingly, distillation of impure PO may comprise extractive distillation of impure PO with an extractive distillation solvent. In an embodiment, the process comprises feeding impure PO to a distillation zone, introducing a separate feed of extractive distillation solvent to a zone at a level higher than the impure PO feed, removing purified PO from the zone as a distillate; , and removing the extractive distillation solvent rich in impurities from the zone as bottoms.

Advantageously, extractive distillation may improve energy saving opportunities by enabling a sharp temperature difference between the stage of the upper region and the bottom region of the distillation zone conforming to a relatively flat temperature profile. When extractive distillation solvents are used, a relatively high bottom temperature may be required to boil the solvent, thereby improving the energy saving opportunities in accordance with the present invention.

In an embodiment, the extractive distillation solvent has a boiling point of at least 70°C, such as at least 100°C. In an embodiment, the extractive distillation solvent is selected from aromatic hydrocarbons, cyclic paraffins, glycols, water and t-butyl alcohol. Preferably, the extractive distillation solvent is an aromatic hydrocarbon having 6 to 12 carbon atoms. Even more preferably, the extractive distillation solvent is alkyl benzene, in particular ethyl benzene.

The relative feed ratio of extractive distillation solvent to impure PO can be determined by one of ordinary skill in the art based on the particular nature of the feed and the configuration of the distillation zone. In an embodiment, the solvent to PO feed ratio may range from 0.1 to 3, preferably from 0.2 to 1.

The impure PO may be obtained by any suitable method. In an embodiment, the impure PO is obtained, for example, by epoxidation of propylene in the presence of a chemical medium such as a peroxide as described above. Preferably, PO is obtained by contacting a peroxide, in particular an organic hydroperoxide, and propylene with a heterogeneous epoxidation catalyst. A product stream comprising PO and alcohol may be withdrawn and optionally subjected to at least one preliminary distillation to recover impure PO as distillate. Most preferably, the organic hydroperoxide is ethylbenzene hydroperoxide. Suitable heterogeneous epoxidation catalysts are known in the art. Preferably, the epoxidation catalyst may comprise titanium and/or inorganic silicates chemically combined with solid silica. Examples of such catalysts and their use in epoxidation are described in EP0345856B1.

The impure PO input to the distillation zone contains significant amounts of PO. In an embodiment, the impure PO comprises in the range of 90 to 99.9% w/w PO, preferably in the range of at least 95% w/w PO.

In an embodiment, the impure PO may comprise in the range of 0.1 to 10% w/w of one or more impurities, preferably up to 5% w/w of one or more impurities. Impurities may include or consist of, for example, water, one or more alcohols, aldehydes, ketones, or combinations thereof. In an embodiment, the impurity comprises at least one of propionaldehyde, acetaldehyde, acetone, methanol and propylene derivatives having 4 to 7 carbon atoms per molecule, in particular derivatives having 5 or 6 carbon atoms per molecule, or made by him However, other impurities may also be present.

In an embodiment, the impure PO comprises propionaldehyde in an amount of at least 250 ppm wt, such as at least 800 ppm or at least 1000 ppm. In an embodiment, the impure PO comprises propionaldehyde in an amount of 5000 ppm or less, such as 3000 ppm or less.

In an embodiment, the impure PO comprises a propylene derivative having 4 to 7 carbon atoms per molecule, in particular a derivative having 5 or 6 carbon atoms per molecule in an amount of at least 50 ppm wt, for example at least 75 ppm do. In an embodiment, the impure PO comprises such derivatives in an amount of 1000 ppm or less, such as 200 ppm or less.

In an embodiment, the impure PO comprises acetaldehyde in an amount of at least 0.1 ppm wt, such as at least 3 ppm. In an embodiment, the impure PO comprises acetaldehyde in an amount of 40 ppm or less, such as 20 ppm or less.

Distillate PO of enhanced purity has a lower concentration of impurities than impure PO and is typically obtained as a side effluent from the distillation zone. In an embodiment, the amount of one or more impurities in the distillate PO is reduced by at least 20% w/w, preferably at least 50% w/w, more preferably at least 90% w/w.

In an embodiment, the distillate PO comprises at least 99.5% w/w PO, preferably at least 99.995% w/w PO. In an embodiment, the distillate PO comprises less than 0.5% w/w impurities, preferably less than 0.005% w/w impurities.

In an embodiment, the distillate PO comprises propionaldehyde in an amount of less than 200 ppm wt, such as less than 100 ppm or less than 50 ppm.

In an embodiment, the distillate PO contains less than 100 ppm wt, for example less than 75 ppm or 50 ppm of propylene derivatives having 4 to 7 carbon atoms per molecule, in particular derivatives having 5 or 6 carbon atoms per molecule. included in an amount less than

In an embodiment, the distillate PO comprises acetaldehyde in an amount of less than 20 ppm wt.

The process is preferably carried out in a continuous mode, ie by maintaining the distillation zone in a substantially steady state for an extended period of time, for example at least 12 hours. Further details of the method, for example the feed rate, can be readily determined by a person skilled in the art.

The distillation zone may in principle be realized or defined by any suitable apparatus or hardware.

From a second aspect, the present invention relates to a distillation system for separating impurities from impure PO, the system comprising:

a structure defining a distillation zone having an inlet for impure PO, a distillate outlet for purified PO, and one or more outlets for a stream rich in impurities;

a bottom reboiler for providing heat to the distillation zone at a first temperature; and

and an intermediate reboiler for providing heat to the distillation zone at a second temperature lower than the first temperature.

Advantageously, the distillation system can be configured for use in or according to any of the methods described above.

The structures defining the distillation zone may be realized or arranged using components known in the art of PO distillation, for example walls, inlets, outlets, conduits, stages and/or packings. The structure may include a column or a plurality of columns.

In an embodiment, the distillation zone comprises at least 30 theoretical plates, preferably at least 40 theoretical plates. Suitably, the total number of theoretical plates may be 100 or less, for example 80 or less single numbers.

In an embodiment, the distillation zone comprises at most 30 theoretical plates, more preferably at most 15 theoretical plates or at most 10 theoretical plates between the bottom reboiler input and the intermediate reboiler input. In an embodiment, the number of such theoretical plates is less than 40% of the total number of stages, preferably less than 20% or even less than 10% of the total number of stages.

In an embodiment, the distillation zone comprises at least 10 theoretical plates, preferably at least 20 theoretical plates, above the intermediate reboiler input.

In an embodiment, the theoretical plates are realized by column stages and/or random packing or structured packing.

In an embodiment, the bottom reboiler is arranged to input heat to the bottom region of the distillation zone and the intermediate reboiler is arranged to input heat to the upper region of the distillation zone. The bottom region and the top region may be defined as above. Thus, ie the intermediate reboiler is positioned above the bottom reboiler in such an arrangement.

The intermediate reboiler may be any heat exchanger or heater capable of transferring heat from the heat source to the upper region of the distillation zone. The intermediate reboiler comprises a first flow path for exiting the liquid mixture from the upper region of the distillation zone to the reboiler, heat exchange or heating means for heating the liquid mixture to a heat source, and a second flow path for returning the heated mixture to the distillation zone. 2 It may include or define a flow path. Intermediate reboilers may include pumps to aid circulation or may rely on natural circulation. Advantageously, the intermediate reboiler may be of the thermosyphon type. However, other types of reboilers are known to those skilled in the art.

The bottom reboiler may be any heat exchanger or heater capable of transferring heat from a heat source to the bottom region of the distillation zone. The bottom reboiler comprises a first flow path for exiting the liquid mixture from the bottom region of the distillation zone to the reboiler, heat exchange or heating means for heating the liquid mixture to a heat source, and a second flow path for returning the heated mixture to the distillation zone. 2 It may include or define a flow path. The bottom reboiler may include a pump to aid circulation or may rely on natural circulation. Advantageously, the bottom reboiler may be of thermosiphon type or kettle type. However, other types of reboilers are known to those skilled in the art.

The distillation zone may be defined exclusively by its structure or by a structure in combination with one or more of the reboilers.

The one or more outlets for the impurity-rich stream may include, for example, one or more of a bottom impurity outlet, a side-draw impurity outlet, and an overhead impurity outlet.

In an embodiment, the system may further comprise a reflux component for recycling the overhead product to the distillation zone.

To comply with extractive distillation, the distillation zone may include an inlet for extractive distillation solvent located above the inlet for impure PO and a bottom outlet for extractive distillation solvent rich in impurities.

In an embodiment, the bottoms reboiler of the system is in the form of a bottoms stripper, which regards the bottoms flow as feed from the main distillation column of the system and returns the overhead vapors of the stripper to the main distillation column. In this embodiment, the intermediate reboiler may be the bottom reboiler of the main distillation column.

From a third aspect, the present invention relates to a process for the preparation of PO, the process comprising the steps of contacting peroxide and propylene with an epoxidation catalyst to obtain PO, for example as described in any part above, and isolating impurities from PO by the method according to the first aspect of the invention and/or using a distillation system according to the second aspect of the invention.

Throughout the description and claims of this specification, the terms “comprises” and “contains” and variations of the terms, such as “comprising” and “comprises,” mean “including, but not limited to,” and , other moieties, additives, components, integers or steps are not excluded. Moreover, the singular includes the plural unless the content otherwise requires: in particular, where the indefinite article is used, it should be understood as contemplating the singular as well as the plural unless the content otherwise requires.

Preferred features of each aspect of the invention may be present as described in conjunction with any of the other aspects. Other features of the invention will become apparent from the following detailed description of the embodiments. In general, the present invention extends to any novelty, or any novel combination of features, disclosed herein (including any appended claims and drawings). Accordingly, a feature, integer, characteristic or compound described in connection with a particular aspect, embodiment or example of the invention may be applied to any other aspect, embodiment or example described herein as long as it is compatible therewith. should understand Moreover, unless otherwise stated, any feature disclosed herein may be replaced by alternative features serving the same or similar purpose.

Where upper or lower limits are exemplified for the sake of property rights, the range of values defined by any combination of any of the upper and lower limits may also be implied.

specific explanation

In order that the present invention may be more readily understood, reference will now be made to the accompanying Figure 1, which shows, for example, a schematic diagram of a distillation column according to one embodiment of the present invention.

1 , a distillation system 2 for separating impurities from impure PO is a wall defining a distillation zone 6 , a bottom reboiler 8 , an intermediate reboiler 10 , and a reflux system 12 . and a column structure 4 therein.

Structure 4 of system 2 defines an impure PO inlet 14 and extractive distillation solvent inlet 16 to distillation zone 6 , and purified PO outlet 18 from distillation zone 6 . . Also, inlets 20 , 22 and outlets 24 , 26 for bottom reboiler 8 and intermediate reboiler 10 , as well as overhead outlet 28 and reflux inlet 30 is limited From bottom to top of column 2, the outlet 24 to the bottom reboiler 8 is at the bottom, followed by the inlet 20 from the bottom reboiler 8, an intermediate reboiler at the same level ( 10) and inlet 22 from the intermediate reboiler, impure PO inlet 14, extraction solvent inlet 16, purified PO outlet 18, reflux inlet 30, and overhead An outlet 28 is present.

A column stage 32 is provided between the inlet and outlet to aid in distillation in the distillation zone 6 . In particular, the column structure 2 comprises 11 theoretical plates between the inlet 20 from the bottom reboiler and the outlet and inlets 26, 22 of the intermediate reboiler, the outlet and inlet of the intermediate reboiler ( 26), (22) comprising 9 theoretical plates between the impure PO inlet 14 and the impure PO inlet 14 and 45 theoretical plates between the impure PO inlet 14 and the extractive distillation solvent inlet 16, and extractive distillation solvent Six theoretical plates are included between the inlet and purified PO outlet (18), and six theoretical plates are included between the purified PO outlet and the overhead outlet (28) and reflux inlet (30). The theoretical plates may be realized in practice using conventional column stages, but may also be implemented as random packing or structured packing in other embodiments.

The bottom reboiler 8 is a thermosiphon reboiler operated by the input of relatively high temperature steam 34 , for example HP or MP steam, and the condensate 36 is discharged. In order to provide an impurity outlet to the distillation zone 6, the bottom reboiler 8 comprises a bleed 40 to draw off the extractive distillation solvent rich in liquid components, in particular impurities.

The intermediate reboiler 10 is a thermosiphon reboiler operated by the input of lower temperature steam 38 , for example LP steam, and the condensate 36 is discharged.

In operation, impure PO and ethyl benzene as extractive distillation solvent are fed to the distillation zone through their inlets. The impure PO includes approximately 99.6% wt of PO, about 3500 ppm wt of propionaldehyde, about 250 pm wt of a propylene derivative having 4 to 7 carbon atoms per molecule, and the remainder other impurities.

The bottom reboiler 8 inputs heat to the bottom region 42 of the distillation zone 6 . The intermediate reboiler 10 inputs heat to the remaining upper region 44 of the distillation zone 6 . The intermediate reboiler represents about 70% of the total reboiler load. The remainder of the reboiler load is occupied by the bottom reboiler (8).

The provided heat causes distillation within the distillation zone. Distillate PO 18 of improved purity, reduced in impurity concentration to greater than 95%, exits the distillation zone as a bottom bleed 40 of impurity rich ethyl benzene exits. The reboiler's reflux system 12 comprising a liquid impurity outlet 48 and a vent 50 is controlled accordingly.

Column 2 is configured such that all successive stages between the inlet 22 from the intermediate reboiler and the purified PO outlet 18 conform to a flat temperature profile, i.e. have stage temperatures that are within the range of 20°C. It works. In particular, the temperature of these stages is maintained in the range of 50° C. to 70° C. for the specific operating pressure under consideration. The intermediate reboiler 10 can thus operate on LP steam 38 having a temperature of about 130° C. and a pressure of about 2.5 bar abs. Such low grade steam 38 may be generated from one or more energy saving measures elsewhere.

In comparison, the maximum temperature of the bottom region 42 is about 170°C. To provide this temperature, the bottom reboiler is operated on MP steam 34 having a temperature of about 220° C. and a pressure of about 18 bar abs.

The ability to use LP steam 34 in the intermediate reboiler 10 lowers the overall demand for HP steam for the bottom reboiler 8 . Additionally, since the number of theoretical plates between the input of the bottom reboiler 8 and the input of the intermediate reboiler 10 is small, the detrimental effect of the intermediate reboiler 10 which supplies some more heat to the column 2 is the effect is reduced.

A person skilled in the art will recognize that a wide variety of modifications of column 2 can be made without departing from the present invention. For example, feed, stage and as a whole column designs can be changed while still accommodating bottom reboilers and intermediate reboilers. In one embodiment, the column is modified with a conventional distillation column operating without extractive distillation solvent to receive the impure PO feed requiring such a setup.

In another embodiment, an existing column, having only a bottoms reboiler, is extended using an additional bottoms stripper in conjunction with the bottoms reboiler, which considers the original bottoms flow as feed and its over in the original column. By returning the head steam, it functions as an original bottom reboiler. The distillation zone of the column thus extends downward. The original bottom reboiler uses a low level of heat and is converted to act as an intermediate reboiler in the new configuration.

Claims (15)

impure propylene oxide comprising propionaldehyde in an amount of at least 250 ppm wt and up to 5000 ppm wt, and a propylene derivative having 4 to 7 carbon atoms per molecule in an amount of at least 50 ppm wt and up to 1000 ppm wt. A method for separating impurities from
wherein said impure propylene oxide is distilled by extractive distillation using an extractive distillation solvent which is an aromatic hydrocarbon having 6 to 12 carbon atoms;
The method comprises distilling impure propylene oxide in a distillation zone to provide distillate propylene oxide of improved purity, wherein heat is provided to the distillation zone by a bottoms reboiler and an intermediate reboiler, wherein the intermediate reboiler is a bottom and providing heat at a lower temperature than reboiler. The method of claim 1 , wherein the heat provided by the intermediate reboiler is from a heat source having a temperature in the range of 80°C to 180°C. The process according to claim 1 , wherein the heat provided by the intermediate reboiler is obtained directly or indirectly from the process stream cooled as part of the process for the production of propylene oxide. 4. The process according to claim 3, wherein the heat provided by the intermediate reboiler is from flash steam produced by cooling the process stream as part of the process for the production of propylene oxide. 3. The process according to claim 1 or 2, wherein the bottom reboiler inputs heat to the bottom region of the distillation zone and the intermediate reboiler inputs heat to the top region of the distillation zone. The process according to claim 5 , wherein the upper region of the distillation zone comprises a plurality of at least 10 consecutive theoretical plates having different stage temperatures of at most 40°C. 7. The method of claim 6, wherein the temperature of the plurality of stages ranges from 50 to 120°C. 6. The method of claim 5, wherein the intermediate reboiler injects heat in a reference stage in the upper region having a temperature at least 50° C. lower than a maximum temperature in the bottom region, the reference stage being a plurality of at least ten consecutive theories in the upper region. a lowermost stage of the stages, wherein the plurality of stages have stage temperatures that differ by at most 40°C. 3. The method according to claim 1 or 2, wherein the intermediate reboiler, or combination of intermediate reboilers, accounts for at least 25% of the total reboiler duty. 3. Process according to claim 1 or 2, wherein the distillate propylene oxide comprises at least 99.5% w/w of propylene oxide. contacting peroxide and propylene with an epoxidation catalyst to obtain propylene oxide; and separating impurities from propylene oxide by the method according to claim 1 or 2 . delete delete delete delete

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